I like linear, it takes the stress out of choosing which boss order you should do, not to mention the time it takes deciding and testing the different bosses. It also makes it easier to track progression world first race.

Is the heroic progression also linear? Can you do a certain "area boss" before the next area, or must every heroic boss require the previous heroic boss killed in order to pull?

should be able to do any boss on heroic then switch to normal for another. This likely won't work for heroic lei'shen (who'll probably need a full heroic clear) and ra-den will definitely need a full heroic clear (including lei'shen)